ATR is used for conveying parameters that are required by the card to establish a data communication pathway. This message is conveyed as soon as the power in the smart card is set to on and stands for answer to reset (ATR).
It is usually up to 33 bytes, contains the transmission parameters such as T = 0 and T = 1, which are supported by the card. It also carries all the necessary information that is required to be known by the host such as:
- Data transmission rate
- Card hardware parameters
- Chip serial number
- Mask version number